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HomeDatabaseMysql TutorialOracle char与varchar2的比较与使用

最近工作处理了一些有关Oracle字符数据类型的问题,特做如下整理,希望和大家共同学习。首先对char与varchar2类型做下比较,区别

最近工作处理了一些有关Oracle字符数据类型的问题,特做如下整理,希望和大家共同学习:

一、首先对char与varchar2类型做下比较:

区别:

  1.CHAR的长度是固定的,而VARCHAR2的长度是可以变化的, 比如,存储字符串“abc",对于CHAR (20),表示你存储的字符将占20个字节(包括17个空字符),而同样的VARCHAR2 (20)则只占用3个字节的长度,20只是最大值,当你存储的字符小于20时,按实际长度存储。

例:

 SQL> create table testchar(c1 char(20), c2 varchar2(20));

表已创建。

SQL> insert into testchar values('dylan','dylan');

已创建 1 行。

SQL> commit;

提交完成。

SQL> select length(t.c1), length(t.c2) from testchar t;

LENGTH(T.C1) LENGTH(T.C2)
------------ ------------
          20            5

  2.CHAR的效率比VARCHAR2的效率稍高。

  3.目前VARCHAR是VARCHAR2的同义词。工业标准的VARCHAR类型可以存储空字符串,但是oracle不这样做,尽管它保留以后这样做的权利。Oracle自己开发了一个数据类型VARCHAR2,这个类型不是一个标准的VARCHAR,它将在数据库中varchar列可以存储空字符串的特性改为存储NULL值。如果你想有向后兼容的能力,Oracle建议使用VARCHAR2而不是VARCHAR。 

何时该用CHAR,何时该用varchar2?

  CHAR与VARCHAR2是一对矛盾的统一体,两者是互补的关系.

  VARCHAR2比CHAR节省空间,在效率上比CHAR会稍微差一些,即要想获得效率,就必须牺牲一定的空间,这也就是我们在数据库设计上常说的‘以空间换效率’。

  VARCHAR2虽然比CHAR节省空间,但是如果一个VARCHAR2列经常被修改,而且每次被修改的数据的长度不同,这会引起‘行迁移’ (Row Migration)现象,,而这造成多余的I/O,是数据库设计和调整中要尽力避免的,在这种情况下用CHAR代替VARCHAR2会更好一些。

二、关于oracle中文字符占用多少存储空间问题

方法一:
一个汉字在Oracle数据库里占多少字节跟数据库的字符集有关,UTF8时,长度为3.

测试一个汉字在各种字符集中到底占用多少字节:

SQL> select    lengthb('汉')    from    dual;

LENGTHB('汉')
-------------
            2

方法二:

查看oracle数据库当前字符集:

SQL> select * from nls_database_parameters where parameter='NLS_CHARACTERSET';

PARAMETER                      VALUE
------------------------------ ----------------------------------------------------------------
NLS_CHARACTERSET               ZHS16GBK


一个英文字母是一个字符,一个中文汉字是几个字符呢?我们知道,一个中文汉字是双字节字符,但它有几个字符与其数据库字符集有关。如果数据库字符集使用单字节US7ASCII,则一个中文汉字是二个字符;如果数据库字符集使用双字节字符集ZHS16GBK,则一个中文汉字是一个字符。有关这一点可以使用 Oracle的函数Substr得到证明。 


使用US7ASCⅡ字符集时: 

SQL> select substr('南京大学',1,2) from dual;

SUBS
----
南 

使用ZHS16GBK字符集时: 

SQL> select substr('南京大学',1,2) from dual;

SUBS
----
南京

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